Distillate fuels produced from non-petroleum sources and derived largely from the Fischer Tropsch (FT) process are typically highly paraffinic and have excellent burning properties and very low sulphur content. This makes them highly suitable as a fuel source where environmental concerns are important; and in circumstances where the security of supply and availability of petroleum supplies may cause concern.
However, although many physical properties for conventional distillate fuels can be matched and even outperformed, the fuels derived from FT processes and the like can not provide conventional jet fuel “drop-in compatibility” (i.e. be amenable to direct substitution within the conventional petroleum-derived jet fuel infrastructure), as they lack some of the major hydrocarbon constituents of typical petroleum-derived kerosene fuel. For example, due to their low aromatic content, FT jet fuels tend not to comply with certain industry jet fuel specified characteristics such as minimum density, seal swell propensity and lubricity.
This difficulty in obtaining suitable jet fuel entirely from non-petroleum feedstocks has triggered several developments in the downstream processing of feedstock in order to obtain suitable products.
For example, U.S. Pat. No. 4,645,585 teaches the production of novel fuels, including jet fuel components, from the extensive hydroprocessing of highly aromatic heavy oils such as those derived from coal pyrolysis and coal hydrogenation.
WO 2005/001002 relates to a distillate fuel comprising a stable, low-sulphur, highly paraffinic, moderately unsaturated distillate fuel blendstock. The highly paraffinic, moderately unsaturated distillate fuel blendstock is prepared from an FT-derived product that is hydroprocessed under conditions during which a moderate amount of unsaturates are formed or retained to improve stability of the product.
U.S. Pat. No. 6,890,423 teaches the production of a fully synthetic jet fuel produced from an FT feedstock. The seal swell and lubricity characteristics of the base FT distillate fuel are adjusted through the addition of alkylaromatics and alkylcycloparaffins that are produced via the catalytic reforming of FT product. This process can result in a suitable aviation fuel generated entirely from a non-petroleum source, but the additional reforming steps required to generate the alkylaromatics and alkylcycloparaffins impart significant additional cost and complexity to the process.
US2009/0000185 teaches a method for producing a jet fuel from two independent blendstocks, where at least one blendstock is derived from a non-petroleum derived feedstock, which may be an FT source. In one form of the described method, the second blendstock is also produced via a non-petroleum source, such as via the pyrolysis or liquefaction of coal. However, the provision of at least two independent synthetic feedstocks is highly problematic and less likely to be cost effective when contrasted with petroleum-based fuel sources.
Accordingly, there remains a strong need for a fully-synthetic (i.e. non-petroleum sourced) aviation fuel and an economical means of producing it.